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Solid

Solid is one of the four common states
of matter. The three others are gas,
liquid, and plasma.
There are also some other exotic states of matter that have been discovered
in recent years.

Unlike liquids and gases, solids have definite shapes. If you pour some milk
into a glass or fill a balloon with helium, the liquid (milk) or gas (helium)
takes on the shape of the container (the glass or balloon). A solid keeps its
own shape.

The atoms
or molecules in a solid are packed
together much more tightly in a solid than in a gas or a liquid. The atoms or
molecules in a solid have fixed positions; they don't move around like atoms
or molecules in a gas or liquid do. That's why liquids and gases can flow,
but solids don't.

Solids are usually much more dense
than liquids and gases, but not always. Mercury,
a metallic element that is
a liquid at room temperature, is denser than many solids. Aerogel,
a very unusual human-made solid, is about 500 times less dense than water.

Many solid materials will melt when heated. When solids melt,
they become liquids. For example, when iron is heated to a temperature
around 1,538° C. (2,800° F.) it melts and becomes molten iron. When
liquids are cooled they can freeze
and become solids. A familiar example is liquid water
turning into ice. Under certain conditions (usually low pressure)
some solids can turn directly into a gas without first melting and becoming
a liquid. When a solid turns directly into a gas we call the change "sublimation".
Maybe you have seen dry ice (which is very cold, frozen carbon
dioxide) sublimate.

Crystals, wood, rocks,
most metals, and glass are all examples
of common solids. Your bones and teeth are also solid materials. That's a good
thing... it would be hard for you to eat or get around if it weren't for solids!

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